Activated carbonaceous material, such as activated carbon, is a porous substance that has a large surface area and accordingly a large adsorption ability for a wide range of uses as an adsorbent for various purposes. For example, activated carbon may be used for adsorbing gases and vapors, recovering solvents, purifying gases, deodorizing gases, and contacting with liquids so as to treat water, decolor or purify solutions. Additionally, activated carbon may be used as carriers for catalysts. On account of its highly non-specific adsorptive properties, activated carbon may be the most widely used adsorbent. Further, statutory requirements as well as increasing environmental awareness are leading to an increasing demand for activated carbon.
Activated carbon is generally obtained by carbonization, such as by pyrolysis, smoldering, or coking, followed by the subsequent activation of suitable carbon-containing starting materials. Carbonaceous starting materials which lead to economically viable yields are preferred, since the weight losses caused by the removal of volatile constituents during carbonization and caused by burn-off during activation are considerable. The condition of the activated carbon produced—finely porous or coarser porous, strong or brittle etc.—depends on the carbon-containing starting material. Examples of standard carbonaceous starting materials include coconut shells, wood waste, peat, hard coal, pitches, but also particular plastics, such as for example sulfonated polymers, which play a major role inter alia in the production of activated carbon in the form of small granules or spheres.
Various forms of carbon may be used: powder coal, splint coal, granular coal, shaped coal, granule carbon, and spherical carbon. Granular coal, in particular spherical coal, is in very great demand for particular application areas, such as for many of the applications noted above. Activated carbon is usually produced in rotary tubular kilns (i.e. rotary tubular furnaces). These have, for example, a location for the raw material charge to be introduced at the start of the kiln and a location for the end product to be discharged at the end of the kiln.
For example, it is known to produce an activated carbon from an organic material with the simultaneous production of a reduced metal or metal oxide. This process heats organic material at a temperature of at least 200° C. while generating combustible distilled gas from the organic material and activating the carbonized material by bringing it into contact with a material containing at least one metal compound at a temperature at which the metal compound is reduced by reaction with carbon. This process is achieved in a rotary kiln where the organic material is charged into one end of the rotary kiln and air or oxygen is charged into the opposite end of the rotary kiln. Another process activates lignite coke in a rotary tube after the addition of 25% aqueous potassium carbonate. These processes both require that the rotary kiln be operated at a temperature of typically 650-850° C. and further require a residence time of 3-5 hours to activate coal, for example.
Another process involves a carbonaceous raw material impregnated with a chemical activating agent both of which are treated by controlling the rate of heat transfer to the particles via indirect heating of the activation furnace and simultaneously introducing a flow of independently controlled sweep gas at spaced intervals along the path of travel. Yet another process that is in use involves a method for the production of spherical activated carbon from powdered carbon material such as soot, bituminous coal, anthracite, charcoal and pitch as binder. A spherical activated carbon is produced by agglomeration, drying, charring and activation. Additionally, another process involves the production of spherical activated carbon from pitch by solvent agglomeration and activation in an ammonia atmosphere at from 550°-1,000° C. and additional steam treatment. A further method involves a process for the production of spherical carbonaceous material and spherical activated carbon from pitch and amorphous coal particles with a viscosity modifier. These processes are expensive in terms of equipment and engineering, since the production of spherical starting materials requires additional process stages. The production of shaped activated carbon with very high adsorption capacity and BET (Brunauer, Emmett, and Teller method) surface area is limited by the reduction in hardness and abrasion resistance as the degree of activation increases, this being due to the nature of the process.
Additionally, the carbonization phases or step of these processes tends to produce amounts of acidic reaction products that tend to be extremely corrosive to the rotary kilns, which then imposes extremely high demands on the corrosion-resistance of the rotary tubular kiln material. Oftentimes, the rotary kilns may be comprised of different and separate compartments or stages to accommodate the corrosive process stage of carbonization being separated from the high-temperature stage of activation so as to prevent having to make a single rotary kiln out of expensive metals that resist the corrosiveness of the carbonization step.
Also, conventional processes use carbonaceous material feedstocks with a wide variety of sizes. For example, the feedstock for conventional processes may vary in size from 1 to 2,000 microns. During activation, a significant percentage of this feedstock is consumed. Another general problem associated with conventional processes is that the activation times are so great, that the processes are not generally responsive to minimal process changes. For example, if it takes 5 hours to activate a carbonaceous material with a conventional process and it is desired to change a characteristic of the activated carbonaceous material, small changes in the residence time is not going to produce such change in characteristics.